Dietary soy consumption has been associated with decreased breast cancer risk in demographic and observational studies in women, and the protective effects appear to be stronger when soy is consumed earlier in life. Rodent studies have demonstrated profound protective effects for animals exposed early in life to soy isoflavones. Our data in the postmenopausal monkey model clearly show antiproliferative effects of soy consumption on the breast, and we have also observed that soy consumption reduces estrogen exposure of the breast. We have demonstrated the predictive value of the monkey model for breast cancer risk in the postmenopausal setting, and we believe that the model is particularly applicable to studies of pubertal breast development, which is difficult to study in human patients. We hypothesize that dietary soy consumption during puberty reduces breast proliferation, induces breast differentiation, and reduces hormonal exposure to the developing breast; such an effect would explain the observed lower risk of breast cancer in women and animals exposed to soy diets early in life. To test this hypothesis, we propose to randomize 40 pre-pubertal female cynomolgus monkeys to receive either a high-soy diet (modeling a soy-supplemented diet) or no dietary soy (modeling a typical North American diet), and to follow them longitudinally through the course of menarche and pubertal breast development by repeated sampling via breast biopsies and serum collection. Our specific aims are: 1) to determine breast tissue responses to soy consumption during puberty; 2) to determine the effects of dietary soy on ovarian hormones during menarche and menstrual cycling; 3) to determine intratissue estrogens and the enzymes that regulate local estrogen production in the breast; and 4) to explore patterns of gene expression in mammary epithelium which may be modulated by soy consumption. The proposed work will provide a unique view of an otherwise inaccessible period of breast development in a primate model with high genetic and physiologic similarity to human beings.